The present invention relates to a cruise control system for an automotive vehicle, commonly referred to as an auto-drive or speed control, which automatically controls the vehicle speed without a driver's operation of the accelerator pedal once the vehicle speed is set to a desired value.
In general, cruise control systems of this kind are roughly divided into those employing a motor type actuator which drives an accelerator link using a motor as a power source and those utilizing a negative pressure type actuator which drives the accelerator link using the intake manifold absolute pressure as a power source. Since the former is disadvantageous in terms of weight, cost and responsinity, the latter, which is free from such defects, has been chiefly employed in recent years. An increase set function is also known which allows an increase in the vehicle speed, without a driver's operation of the accelerater pedal, only by keeping on pressing a switch called an increase set switch and which permits a constant speed drive thereafter at the speed of when the increase set switch was released. With the conventional cruise control system employing the aforementioned negative pressure type actuator, such a function cannot be sufficiently performed because the desired acceleration cannot be obtained when the driver desires to increase the vehicle speed. That is to say, in this kind of system, acceleration is carried out by introducing the intake manifold absolute pressure 100 percent into the actuator during increase, but since the intake manifold absolute pressure rises with an increase in the throttle valve opening, the actuator generating power gets into equilibrium with the reaction force of the accelerator link at a midpoint of the full stroke, making it impossible to develop an acceleration that meets the driver's requirement.
Another function of the cruise control system is a resume function which, even after releasing a constant speed driving operation by operating a brake switch or the like, allows the vehicle speed to return to its originally set speed, without involving an operation of the accelerator pedal, only by manipulating a switch called a resume switch. With the conventional cruise control system employing the negative pressure type actuator, however, such a function cannot fully be fulfilled because the vehicle speed cannot be restored to the set speed within a desired period of time. Namely, with the cruise control system of this type, in the case where the vehicle speed at the time of operating the resume switch is more than about 10 Km/h below the set speed, the intake manifold absolute pressure is introduced 100 percent into the actuator, but the desired acceleration cannot be developed for the same reason as mentioned previously; namely, much time is required for restoring the vehicle speed to the set one, thus failing to satisfy the driver's demand for a sporty drive.
Furthermore, in the case of ascending a long slope of, for instance, about five percent, using overdrive, since the vehicle speed gradually lowers even if the throttle valve is opened full, a steady speed drive at a desired speed cannot be accomplished. To prevent the reduction of the vehicle speed while ascending a slope, a certain kind of cruise control system is designed so that when the vehicle speed has become lower than the set speed by a predetermined value, for instance, 7 Km/h, the overdrive is released and the transmission is automatically changed over, for example, from fourth gear to third to restore the vehicle speed, and so that when the vehicle speed has returned substantially to the set speed, for instrance, about 3 Km/h below the set speed, the overdrive is resumed. With such a method, however, the changeover cycle from the overdrive to the immediately lower gear of the transmission or vice versa may sometimes become extremely short according to the gradient of the slope, so that the vehicle speed is frequently altered, resulting in a very uncomfortable drive. Since the cruise control system is primarily intended to reduce the fatigue of long, high-speed driving, factors providing such uncomfortable driving must be eliminated as much as possible. A solution to this problem is, for example, to release the overdrive while ascending a slope and to continue driving at the immediately lower gear. In this case, however, it is technically difficult to automatically release the overdrive, while detecting whether the slope is upward or downward and its gradient; in addition, driving at the immediately lower speed throughout the ascending of the slope consumes fuel, and hence is uneconomical.